This is the May 2015 wireless display update for the Xbox One! I tested the preview with both my Surface 3 tablet as well as my Android phone (Samsung Galaxy S5). No extra hardware required, the Xbox One now fully supports Miracast.
If you don't have the preview but want Miracast on your Xbox One now, this is my favorite accessory on Amazon:
Netgear Miracast Adapter:
If you have the preview, you can get the Wireless Display app via the Quest titled "Extend your screen." Start the quest to be taken to the Xbox Store page to start your download. Once it's finished you can launch the quest again to launch the app and start working through the scenarios we'd love for you to try.
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